The present invention relates to a control system for a variable frequency generator. Such a system may be used with an aircraft generator driven, for example, by a gas turbine engine.
A known type of generator for use with an aero-engine is disclosed in GB 1 596 291. The generator comprises three stages, each of which has a rotor and a stator with the rotors being attached to a common drive shaft. The first stage or pilot exciter comprises a permanent magnet generator having a permanent magnet rotor co-operating with three phase stator windings. The stator windings are connected via a rectifier and voltage regulator to the main exciter winding forming the stator of the main exciter. The rotor of the main exciter comprises three phase windings which are connected via a rectifier to a rotor winding of the main generator stage, which has three phase stator windings connected to the generator output. This arrangement is also applicable to variable frequency machines and DC output machines where the main generator stator winding is connected via a rectifier to give a DC generator output.
The output voltage of the generator is regulated by comparing the output voltage with a reference and controlling the voltage regulator in order to control the main exciter field. The generator must be designed to maintain the nominal output voltage for the full electrical load throughout the operating speed range which may typically be 380 to 780 Hz.
A particular problem experienced in the operation of generators of the above-mentioned type driven at variable speed, is the maintenance of stability of control systems which are provided to facilitate maintaining the transient and steady state voltages within permissible levels as the load and the drive speed varies. A known control system of this type has been used to regulate generator operating parameters relating to average generator output voltage, high phase take-over and generator output current. Signals corresponding to each of these parameters are passed through a highest wins circuit, and the winning signal is subject to stability compensation. Stability compensation is provided by a negative feedback signal from the output of the main exciter field drive, the signal being fed back through a stability compensation circuit.
In accordance with the invention, there is provided a control system for a variable frequency generator of the type comprising a main stator winding for providing a generator output, excited by a main rotor winding, and a main exciter rotor winding for energising the main rotor winding, excited by a main exciter field winding, the control system comprising: a plurality of inputs adapted to receive respective signals corresponding to respective generator operating parameters; an output adapted to drive the main exciter field winding and a plurality of negative feedback loops between the control system output and the inputs, each loop including a respective stability compensation network operable to process a feedback signal corresponding to one of said operating parameters. This facilitates appropriate tailoring of the stability compensation network characteristics to process each type of signal in an optimal manner.
An operating characteristic of at least one of the stability compensation networks may be adaptively controllable in accordance with a present operating frequency of the generator.
Conveniently, the or each of the adaptively controllable networks may be adaptively controllable in accordance with a predetermined frequency range in which the present operating frequency falls.
The control system may include sensing means operable to sense a plurality of generator operating parameters and to provide to the respective inputs respective signals corresponding to the operating parameters.
The sensing means may be operable to provide signals representing average generator output voltage and/or high phase take-over and/or generator output current.
At least one of the networks may include a switch operable by a frequency dependent input signal. The arrangement may be such that, in use, one of said signals corresponding to a generator operating parameter is directed along a first route through the network until the switch is operated so as to cause the signal to be redirected along a different route through the network. In this manner, the signal corresponding to a generator operating parameter can be caused to pass through a selected arrangement of network components appropriate for providing a desired level of stability compensation in accordance with the present operating frequency. Improved stability control is thereby facilitated across a range of operating frequencies.
The, or each switch may be arranged to operate at a respective predetermined operating frequency.
Each said network may be provided with a respective said switch, the switches being arranged to operate at a common predetermined operating frequency.
The control system may be operable to provide signals respectively corresponding to the average voltage of each phase of the generator output and to provide to one of said control system inputs a high phase take-over signal corresponding to the highest of the sensed average voltages. This is advantageous over the use of a peak voltage sensing circuit for HPT regulation, as described for example in GB-A-2293704, since the discharge rate of a peak sensing circuit can give different values of ripple on the sensed voltage at different frequencies causing changing levels of regulated voltage as the load increases. Also, variations in the peak to RMS ratio of the generator output waveform can occur over the full load and power factor range in a variable frequency control system, and the regulation level is required to remain constant under different operating conditions. Regulators operating on peak sensing give varying RMS levels of regulation as the ratio varies.
In accordance with another aspect of the invention, a control system for a variable frequency generator of the type comprising a main stator winding for providing a generator output, excited by a main rotor winding, and a main exciter rotor winding for energising the main rotor winding, excited by a main exciter field winding, the control system comprising an input adapted to receive a signal corresponding to a generator operating parameter; an output adapted to drive the main exciter field winding and a negative feedback loop between the control system output and the input, the loop including a stability compensation network operable to process the signal, an operating characteristic of the stability compensation network being adaptively controllable in accordance with a present operating frequency of the generator.